There are numerous situations in which composite material is molded or formed to provide high strength, lightweight items or components that exhibit a desired geometry. One application of increasing importance is the fabrication of aircraft components such as wing and stabilizer panels that are formed from thermosetting resins and high strength anisotropic fibers such as graphite. Typically, the panels are formed to exhibit a desired surface contour by placing layers of uncured composite material on a mold having an upper surface that is machined or otherwise shaped to match the contour to be established in the finished panel. The laid up composite material is then cured by, for example, placing the mold in an autoclave.
To be suitable for use in an aircraft, the surface contour of the fabricated panel cannot deviate from the contour of the mold by more than a specified amount. Although the surface area of such panels often is relatively large, a contour tolerance on the order of a few hundredths of an inch may be imposed.
Since the surface of the mold is machined or otherwise formed to reflect the desired contour of the fabricated composite panels, several attempts have been made to use the mold for dimensional inspection of the panel surface. For example, attempts have been made to use feeler gauges to measure gaps between the mold and the surface of the panel that can be caused by warpage during the curing process. However, feeler gauges are not suitable for inspecting surface contours of large composite panels because they only can be used along the periphery of the panel to measure gaps between the panel and the mold.
Another prior technique that has been used in an attempt to measure the thickness of gaps between a composite panel and the mold uses a conventional eddy current sensor and an ultrasonic thickness gauge. The eddy current sensor is positioned at a number of predetermined locations on the surface of the composite panel that faces away from the mold and is used to determine the distance between that panel surface and the surface of the mold that determines the panel contour. The ultrasonic thickness gauge also is placed on the surface of the composite panel that faces away from the mold at the same locations as the eddy current sensor and determines the thickness of the panel. If the local thickness of the composite panel is uniform, the difference between the two measurements is indicative of gaps (and hence warpage) at the measurement locations. Because this technique is based on measurements between the mold and the surface of the composite panel that faces away from the mold, it often is not usable in situations in which the surface of the composite panel that faces the mold must closely conform to the mold contour without a corresponding requirement on the contour or smoothness of the other panel surface. Further, the eddy current sensor/ultrasonic thickness gauge technique cannot be employed in situations in which the entire mold or the surface of the mold is formed of a nonconductive material (e.g., a composite material). A further disadvantage or drawback with respect to relatively large panels, such as wing and stabilizer panels, is that mechanical arms or other structure must be used to position the eddy current sensor and ultrasonic gauge at locations that cannot otherwise be reached by operating personnel. For all these reasons, the eddy current sensor/ultrasonic gauge technique often is not desirable from the standpoint of complexity, equipment cost, and the amount of time required to perform the desired dimensional inspection.
U.S. Pat. No. 4,703,648, which is assigned to the assignee of this invention, discloses a technique that at least partially overcomes disadvantages and drawbacks of prior techniques used to measure the thickness of gaps between a composite panel and the mold utilized to form the composite panel. In the technique disclosed in U.S. Pat. No. 4,703,648, the panel is first removed from the mold and elongate, inelastically deformable gauging strips are positioned on the surface of the mold at locations at which conformity between the mold contour and the contour of the panel are to be determined. Also placed on the surface of the mold is a series of inflatable tubes that are positioned at locations that do not interfere with the positioning of the inelastically deformable gauging strips. To perform the gauging operation, the tubes are inflated and the composite panel is placed on top of the tubes in a position that is spaced apart from and above the position in which the panel was molded. The tubes then are deflated to slowly lower the panel so that the molded surface thereof presses downwardly on and flattens or crushes the inelastically deformable gauging strips. When the panel has settled into a position in which it is supported only by the gauging strips, the tubes are reinflated to lift the panel off the gauging strips. The panel is then removed and the thickness of the gauging strips is measured to thereby determine panel warpage at the desired measuring points.
Although the technique disclosed in U.S. Pat. No. 4,703,648 at least partially alleviates disadvantages and drawbacks of prior techniques, a need exists for improved warpage gauging methods and apparatus. For example, placement of the inelastically deformable gauging strips and the inflatable tubes must be carefully executed and, thus, can consume a substantial amount of time. Inflation of the tubes, replacement of the panel in its proper position and the subsequent steps of deflating the tubes and removing the panels also is a time-consuming process. The final step of measuring the thickness of the deformed gauging strips also is a time-consuming process that must be executed with care. Further, if the measurement results are not conclusive (e.g., the results indicate panel regions at or very near the allowed contour tolerance), the panel cannot be reinspected without repeating the entire measurement process.
One prior technique that has been utilized in a different dimensional inspection application relates to the use of strain gauges to detect and measure abnormalities in the inner diameter of rigid, precisely dimensioned tubing (e.g., detect and measure dents in the tubing wall). More specifically, U.S. Pat. No. 4,235,020 discloses a cylindrical scanner or sensor that includes eight spring fingers that are equally spaced about the circumference of the scanner and extend radially outward. Located on each spring finger is a pair of strain gauges that are positioned so that radial deflection of the spring finger results in changes in the electrical resistance of the strain gauges. The scanner is dimensioned so that the spring fingers bear against and are urged radially inward by the inner wall of the tube to be inspected. To detect and measure dents or other abnormalities in the inner diameter of the tubing, the sensor is pulled through the tube at a predetermined uniform speed by a cable. Dents or other irregularities in the inner wall of the tube cause spring fingers that contact the irregularity to deflect which, in turn, causes a change in the resistance of the associated pair of strain gauges. Since the eight pair of strain gauges can be connected in a variety of manners, including arranging four strain gauges of two selected spring fingers to form a conventional bridge circuit, deviations in the diameter of the tube can be measured. Since the speed at which the sensor is pulled through the tube is uniform and known, the location of the dimensional irregularity also can be detected.
The sensors of the type disclosed in U.S. Pat. No. 4,235,020 are not suitable for use in measuring the gap between two planar surfaces. Specifically, those devices are designed to be contained by the inner wall of a tube or other structure so that the orientation of the sensor within the tube remains constant as the sensor is pulled through the tube. Thus, the spring fingers are deflected only by dents or other irregularities in the inner wall of the tube. In contrast, if such a sensor were drawn along the gap between two contoured planar surfaces, the sensor would be free to twist or rotate and free to tilt up or down relative to the direction of travel. With a sensor of the type disclosed in U.S. Pat. No. 4,235,020, any such changes in physical orientation of the sensor would cause at least some deflection of the spring fingers, thus affecting the resistance of the strain gauges and causing measurement error. Since warpage of composite panels such as aircraft wing and stabilizer panels often is limited to a few hundredths of an inch, reliable inspection is not possible unless the sensor is accurate within a few thousandths of an inch. Sensors of the type disclosed in U.S. Pat. No. 4,235,020, include no provision that would result in the required measurement accuracy in the composite panel warpage measurement environment.